In loading and unloading cargo on and off large transport aircraft, maximum efficiency has been obtained through end loading, i.e., loading and unloading through one end of the aircraft. While front end loading has been utilized, more often the aft or tail of the fuselage is the preferred location for a cargo door assembly. Such end loading eliminates turning and maneuvering operations otherwise necessary; however, special problems are created in the design and construction of the fuselage adjacent the fuselage as well as in the design, construction and operation of the closures or doors for such openings.
For ground loading and unloading, a simple hinged door or doors that swing outwardly have been employed in order to gain unobstructed access to the interior of the aircraft at or near its maximum transverse dimension. As the size of the aircraft increases, this becomes more difficult since the open door must be properly supported to withstand not only its own weight, but also air loads and winds to which it may be subjected. In the past this has been satisfied largely through the use of doors that consist of multiple sections, such as the so-called clam shells or the like. Although this type of construction increases the overall complexity of the door and its operation, it has been the preferred approach rather than add weight to the aircraft by reinforcement of the door and the adjacent supporting structure. However, this added complexity in structure and operation can lead to ineffective operation, high maintenance requirements and increased failure rates.
In modern day high speed cargo aircraft, it is desirable to provide substantially straight-in loading and unloading of cargo on the ground as well as the capability of aerial delivery of cargo loads as large as the effective cross-sectional area of the cargo compartment. When greater performance of this type is required, the above-noted design problems are compounded. Large openings make it necessary to reinforce the adjacent stationary structure to assure its integrity during the aerial delivery operation, and hence add weight to the aircraft. With multiple section door assemblies, the extreme contour condition of the fuselage end has resulted in relatively complex structures and mechanisms for synchronizing the movement of the door sections. Often times, this requires the use of numerous limit switches which reduce the reliability of operation and increase the opportunity for malfunction.
In addition to the aforementioned considerations, pressurization of the aircraft interior during high altitude flights presents further difficulties such as the pressurization of unusable volumes of space plus leakage through cargo doors in the outer fuselage structure. In order to maintain the cargo door in a closed position during pressurization, gang locking arrangements are employed. One particular problem with the use of gang locking arrangements is the inability of the operator to obtain a visual inspection of the condition of the lock. This requires the reliance on mechanical devices to assure that a locked condition has been achieved, thus increasing complexity and reducing reliability. The failure to adequately lock the cargo door of a pressurized aircraft can have disastrous results if a door should blow-out which can cause extensive damage to the aircraft it not a fatal crash.